U.S. patent application number 10/781412 was filed with the patent office on 2004-08-26 for retainer, exposure apparatus, and device fabrication method.
Invention is credited to Murasato, Naoki.
Application Number | 20040165287 10/781412 |
Document ID | / |
Family ID | 32866368 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040165287 |
Kind Code |
A1 |
Murasato, Naoki |
August 26, 2004 |
Retainer, exposure apparatus, and device fabrication method
Abstract
A retainer for holding an optical element includes a detector
for detecting a deformation amount of the optical element, and an
adjustment unit for adjusting the deformation of the optical
element based on the deformation amount.
Inventors: |
Murasato, Naoki; (Tochigi,
JP) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
345 PARK AVENUE
NEW YORK
NY
10154
US
|
Family ID: |
32866368 |
Appl. No.: |
10/781412 |
Filed: |
February 17, 2004 |
Current U.S.
Class: |
359/819 |
Current CPC
Class: |
G02B 27/0068 20130101;
G02B 7/023 20130101 |
Class at
Publication: |
359/819 |
International
Class: |
G02B 006/26; G02B
006/42; G02B 007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 17, 2003 |
JP |
2003-037565 |
Claims
What is claimed is:
1. A retainer for holding an optical element, said retainer
comprising: a detector for detecting a deformation amount of the
optical element; and an adjustment unit for adjusting the
deformation of the optical element based on the deformation
amount.
2. A retainer according to claim 1, wherein said detector is a
strain gauge.
3. A retainer according to claim 1, wherein said detector is
arranged on the optical element.
4. A retainer according to claim 1, wherein three detectors are
arranged on the same circumference at a pitch of 120.degree..
5. A retainer according to claim 1, wherein said adjustment unit
equalizes a load applied to the optical element.
6. A retainer according to claim 1, wherein said adjustment unit
adjusts the load applied to the optical element to reduce
aberration of the optical element.
7. A retainer according to claim 5, wherein-said adjustment unit
includes a coil spring.
8. A retainer according to claim 7, wherein said adjustment unit
includes an adjustment screw for adjusting a length of the coil
spring.
9. A retainer according to claim 1, wherein three adjustment units
are arranged on the same circumference at a pitch of
120.degree..
10. A retainer according to claim 1, further comprising a support
part that supports the optical element at approximately three
points.
11. A retainer according to claim 1, wherein there are three
detectors and three support parts, wherein each detector is
arranged between two adjacent supports parts.
12. A retainer according to claim 1, wherein there are three
adjustment units and three support parts, wherein each detector is
arranged between two adjacent supports parts.
13. A retainer according to claim 1, wherein the number of
adjustment units is more than the number of detectors, and said
adjustment units are driven based on detection results by said
detectors.
14. A retainer according to claim 1, further comprising a support
part that supports the optical element at approximately three
points, wherein said adjustment unit is provided every space
between adjacent two points among the three points, and said
detector is located at least one of the spaces among the three
points.
15. A retainer according to claim 1, wherein said detector and said
adjustment unit are integrated with each other.
16. A retainer according to claim 1, wherein the adjustment unit
includes a component, and said detector detects the deformation
amount by using the component in said adjustment unit.
17. A retainer according to claim 16, wherein the adjustment unit
includes a component, and said detector detects the deformation
amount by measuring a strain amount of the component in said
adjustment unit.
18. A retainer for holding an optical element, said retainer
comprising: an adjustment unit for adjusting a shape of the optical
element, said adjustment unit including a component; and a detector
for detecting a deformation amount of the component in said
adjustment unit, said adjustment unit adjusting the shape of the
optical element based on a detection result by said detector.
19. A retainer according to claim 18, further comprising a support
part for supporting the optical element at approximately three
points, and said adjustment units are arranged at intervals of the
approximately three points.
20. An adjustment method for adjusting a shape of the optical
element into a desired shape, said method comprising the steps of:
obtaining the shape of the optical element; calculating a force to
be applied to the optical element to correct the shape of the
optical element into the desired shape; and applying the force
calculated by said calculating step to the optical element.
21. An adjustment method according to claim 20, further comprising
the steps of: detecting wave front aberration of the optical
element; and applying the force to the optical element so that the
wave front aberration falls within a permissible range.
22. An exposure apparatus comprising: a retainer for holding an
optical element, said retainer including a detector for detecting a
deformation amount in a shape of the optical element, and an
adjustment unit for adjusting the shape of the optical element
based on the deformation amount; and an optical system for exposing
a pattern formed on a mask or reticle onto an object through the
optical element held by the retainer.
23. An exposure apparatus comprising: a retainer for holding an
optical element, said retainer including an adjustment unit for
adjusting a shape of the optical element, said adjustment unit
including a component, and a detector for detecting a deformation
amount of the component in said adjustment unit, the adjustment
unit adjusting the shape of the optical element based on a
detection result by said detector; and an optical system for
exposing a pattern formed on a mask or reticle onto an object
through the optical element held by the retainer.
24. A device fabrication method comprising the steps of: exposing a
pattern on a mask, onto an object by using an exposure apparatus
that includes a retainer that includes three support parts for
supporting an optical element, a first unit for applying a first
elastic force to the optical element in an anti-gravity direction,
and a second unit, arranged opposite to the first unit through the
optical element, for applying a second elastic force to the optical
element in a gravity direction, and an optical system for exposing
a pattern formed on a mask or reticle onto an object through the
optical element held by the retainer; and developing the object
that has been exposed.
25. A device fabrication method comprising the steps of: exposing a
pattern on a mask, onto an object by using an exposure apparatus
that includes a retainer for holding an optical element, said
retainer including an adjustment unit for adjusting a shape of the
optical element, said adjustment unit including a component, and a
detector for detecting a deformation amount of the component in
said adjustment unit, the adjustment unit adjusting the shape of
the optical element based on a detection result by said detector,
and an optical system for exposing a pattern formed on a mask or
reticle onto an object through the optical element held by the
retainer; and developing the object that has been exposed.
Description
[0001] This application claims a benefit of priority based on
Japanese Patent Application No. 2003-037565, filed on Feb. 17,
2003, which is hereby incorporated by reference herein in its
entirety as if fully set forth herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to precision
machines for mounting an optical element, and more particularly to
a projection optical system in an exposure apparatus, etc. The
present invention is suitable, for example, for a retainer that
holds an optical element for a more precise imaging relationship in
an exposure apparatus in projecting and exposing an image on an
original sheet, such as a mask or reticle (these terms are used
interchangeably in this application) onto an object, such as a
single crystal substrate for a semiconductor wafer, a glass plate
for a liquid crystal display (LC-D). The exposure apparatus is used
to fabricate a semiconductor device, an image pick-up device (such
as a CCD), and a thin film magnetic head.
[0003] The fabrication of a device using the lithography technique
has employed a projection exposure apparatus that uses a projection
optical system to project a circuit pattern formed on a mask onto a
wafer and the like, thereby transferring the circuit pattern. The
projection optical system enables diffracted beams from the circuit
pattern to interfere on the wafer and the like, so as to form an
image.
[0004] The devices to be mounted on electronic apparatuses should
be highly integrated to meet recent demands for miniaturization and
low profile of electronic apparatuses, and finer circuit patterns
to be transferred or higher resolution have been demanded
increasingly. A short wavelength of a light source and an increased
numerical aperture ("NA") in a projection optical system are
effective to the high resolution as well as a reduced aberration in
the projection optical system.
[0005] An optical element, such as a lens and a mirror, when
deforming in an projection optical system causes aberration because
an optical path refracts before and after the deformation and light
that is supposed to form an image at one point does not converge on
one point. The aberration causes a positional offset and
short-circuits a circuit pattern on a wafer. On the other hand, a
wider pattern size to prevent short-circuiting is contradictory to
a fine process. Therefore, a projection optical system with small
aberration should hold its optical element(s) without changing a
shape and a position relative to the optical axis of the optical
element in the projection optical system so as to maximize the
original optical performance of the optical element. In particular,
the projection lens tends to have a larger caliber and a larger
lens capacity due to the recent high NA in the projection optical
system, and easily deforms by its own weight. In addition,
diffraction optical elements, which have been eagerly studied
recently, also tend to deform due to its thinness.
[0006] Accordingly, a conventional retainer has used a screw ring
or ball pushing to compress and fix the top of an optical element
that has been supported at its entire peripheral by a metal frame,
or supported the optical element at three points at regular
intervals in its circumferential direction.
[0007] Alternatively, Japanese Patent Application Publication No.
11-149029 proposes a retainer that supports an optical element
using three optical-element support members provided at regular
intervals in its circumferential direction, and elastic members
that push up against the gravity the circumferential part of the
optical element supported by the support members.
[0008] A metal-frame support of the entire peripheral of the
optical element actually results in three point supports or
contacts at irregular angles, because the metal frame that contacts
the optical element has an undulated support surface unfit for the
optical element's contact surface shape. As a result, this support
transforms the optical element's surface, and deteriorates the
optical element's optical.
[0009] On the other hand, the three points supports of the optical
element at regular interval in a circumferential direction cause
optical element's own weight deformations at these three points of
the support members. In particular, a large aperture lens deforms
significantly by its own weight, undesirably deteriorating its
optical performance.
[0010] Moreover, the retainer proposed in Japanese Patent
Application Publication No. 11-149029, varies forces applied to the
elastic members that push up the optical element against gravity or
forces directly applied to the optical element according to
material's physical performance and size, and has a difficulty in
precise control over the relative accuracy among the forces applied
to the elastic members. Thereby, the non-uniform loads applied to
respective elastic members deform the optical element.
[0011] In other words, the conventional optical-element retainers
have not yet contributed to a high-resolution projection optical
system with less aberration which prevents optical element's
deformations which would otherwise cause deteriorated imaging
performance, and meets fine processing requests.
BRIEF SUMMARY OF THE INVENTION
[0012] Accordingly, it is an exemplified object of the present
invention to provide a retainer, an exposure apparatus, a device
fabrication method which may provide desired optical performance by
reducing aberration due to a deformation of an optical element
which otherwise deteriorates the imaging performance.
[0013] A retainer of one aspect according to the present invention
for holding an optical element includes a detector for detecting a
deformation amount of the optical element, and an adjustment unit
for adjusting the deformation of the optical element based on the
deformation amount.
[0014] The detector may be a strain gauge. The detector may be
arranged on the optical element. Three detectors may be arranged on
the same circumference at a pitch of 120.degree.. The adjustment
unit may equalize a load applied to the optical element. The
adjustment unit may adjust the load applied to the optical element
to reduce aberration of the optical element. The adjustment unit
may include a coil spring. The adjustment unit may include an
adjustment screw for adjusting a length of the coil spring. Three
adjustment units may be arranged on the same circumference at a
pitch of 120.degree.. The retainer may further include a support
part that supports the optical element at approximately three
points.
[0015] There may be three detectors and three support parts,
wherein each detector is arranged between two adjacent supports
parts. There may be three adjustment units and three support parts,
wherein each detector is arranged between two adjacent supports
parts. The number of adjustment units may be more than the number
of detectors, and the adjustment units are driven based on
detection results by the detectors. The retainer may further
include a support part that supports the optical element at
approximately three points, wherein the adjustment unit is provided
every space between adjacent two points among the three points, and
the detector is located at least one of the spaces among the three
points. The detector and the adjustment unit may be integrated with
each other. The adjustment unit may include a component, and the
detector may detect the deformation amount by using the component
in the adjustment unit. The adjustment unit may include a
component, and the detector may detect the deformation amount by
measuring a strain amount of the component in the adjustment
unit.
[0016] A retainer of another aspect according to the present
invention for holding an optical element includes an adjustment
unit for adjusting a shape of the optical element, the adjustment
unit including a component, and a detector for detecting a
deformation amount of the component in the adjustment unit, the
adjustment unit adjusting the shape of the optical element based on
a detection result by the detector. The retainer may further
include a support part for supporting the optical element at
approximately three points, and the adjustment units may be
arranged at intervals of the approximately three points.
[0017] An adjustment method of still another aspect according to
the present invention for adjusting a shape of the optical element
into a desired shape includes the steps of obtaining the shape of
the optical element, calculating a force to be applied to the
optical element to correct the shape of the optical element into
the desired shape, and applying the force calculated by the
calculating step to the optical element. The adjustment method may
further include the steps of detecting wave front aberration of the
optical element, and applying the force to the optical element so
that the wave front aberration falls within a permissible
range.
[0018] An exposure apparatus of another aspect according to the
present invention includes the above retainer, and an optical
system for exposing a pattern formed on a mask or reticle onto an
object through the optical element held by the retainer.
[0019] A device fabrication method of another aspect of the present
invention includes the steps of exposing a pattern on a mask, onto
an object by using the above exposure apparatus, and developing the
exposed object. Claims for the device fabrication method that
exhibits operations similar to those of the above exposure
apparatus cover devices as their intermediate products and finished
products. Moreover, such devices include semiconductor chips such
as LSIs and VLSIs, CCDs, LCDs, magnetic sensors, thin-film magnetic
heads, etc.
[0020] Other objects and further features of the present invention
will become readily apparent from the following description of the
embodiments with reference to accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic, partially sectional, perspective view
of an inventive retainer that holds an optical element.
[0022] FIG. 2 is a schematic, partially sectional, perspective view
of the inventive retainer without the optical element.
[0023] FIG. 3 is a schematic sectional view of one example of an
adjustment unit shown in FIG. 1.
[0024] FIG. 4 is a schematic sectional view of another example of
the adjustment unit shown in FIG. 1.
[0025] FIG. 5 is a circuitry of a bridge circuit that uses a strain
gauge.
[0026] FIG. 6 is a schematic sectional view of a variation of the
adjustment unit shown in FIG. 1.
[0027] FIG. 7 is a schematic sectional view of an exemplary
adjustment unit that has an improved contact portion between a
retaining member and a hook.
[0028] FIG. 8 is a schematic sectional view of an exemplary
adjustment unit that has an improved contact portion between the
retaining member and the hook.
[0029] FIG. 9 is a flowchart for explaining an adjustment method
for adjusting a shape of an optical element held by the retainer
shown in FIG. 1.
[0030] FIG. 10 is a schematic perspective view of a variation of
the retainer shown in FIG. 1.
[0031] FIG. 11 is a schematic sectional view of a retainer that
holds an optical element as a mirror.
[0032] FIG. 12 is a schematic block diagram of an exposure
apparatus of one aspect according to the present invention.
[0033] FIG. 13 is a schematic, partial sectional, perspective view
in a lens barrel in the inventive exposure apparatus.
[0034] FIG. 14 is a flowchart for explaining a method for
fabricating devices (semiconductor chips such as ICs, LSIs, and the
like, LCDs, CCDs, etc.).
[0035] FIG. 15 is a detailed flowchart for Step 4 of wafer process
shown in FIG. 14.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] Referring now to accompanying drawings, a description will
be given of an illustrative retainer 100 and exposure apparatus 200
of the present invention. However, the present invention is not
limited to these embodiments, and each element may be replaced
within a scope of this invention. For example, although the
retainer 100 is applied to a projection optical system 230 in the
exposure apparatus 200 in the instant embodiment, it is applicable
to an illumination optical system 214 in the exposure apparatus 200
and other known optical systems.
[0037] Here, FIGS. 1 and 2 are schematic, partially sectional,
perspective views of the inventive retainer 100 applied to the
projection optical system 230 in the exposure apparatus 200,
wherein FIG. 1 is a view of the retainer 100 that has an optical
element 110, and FIG. 2 is a view of the retainer 100 without the
optical element 110. The gravity direction is parallel to the
optical axis of the optical element 110, as shown in an arrow
direction in FIG. 1. When the optical axis of the optical element
is parallel to the gravity direction, the gravity deformation of
the optical element becomes the largest and the gravity deformation
often affects the optical performance. Therefore, the instant
embodiment applies the present invention to a case where the
optical axis of the optical element 110 is parallel to the gravity
and anti-gravity directions. Like elements in respective figures
are designated by the same reference numerals, and a description
thereof will be omitted.
[0038] The retainer 100 includes, as shown in FIGS. 1 and 2, a
retaining member 120 and an adjustment unit 130, holds the optical
element 110, and controls 30 deformations in the optical element
110.
[0039] The optical element 110 is mounted on the retaining member
120, which will be described later, and images light using
reflections, refractions, diffractions, etc. The optical element
110 is supported by a support part 122 in the retaining member 120,
and jointed with the retaining member 120 via a joint part 124 over
its circumference. The optical element 110 includes, for example, a
lens, a parallel plate glass, a prism, a mirror, and a Fresnel zone
plate, a kinoform, a binary optics, a hologram, and other
diffraction optical elements.
[0040] The retaining member 120 provides three support parts 122
for holding the optical element 110 in the gravity direction, at a
pitch of 120.degree. at the outermost peripheral outside an
optically used effective area of the optical element 110, and
mounts the optical element 110 on these support parts 122. The
retaining member 120 is a ring member formed around the optical
axis, and made, for example, of copper alloy, such as brass,
stainless steel, iron, low thermal expansion metal, such as Invar
alloy, carbon steel, and ceramic.
[0041] A support surface 122a of the support part 122 provided in
the retaining member 120 or a surface that contacts the optical
element 110 preferably has a small area not to damage the optical
element 110 in placing the optical element 110 on the retaining
member 120 so that a simulated value is substantially the same as
the estimated deformation amount of the optical element 110 which
occurs due to the optical element 110's own weight after the
optical element 110 is placed on the retaining member 120.
[0042] The joint part 124 is located on an inner surface of the
retaining member 120, and, secures the optical element 110 and the
retaining member 120 along the circumference of the optical element
110. While the instant embodiment uses adhesive for joints, a
mechanical joint, such as a flat spring, can be used for
joints.
[0043] The adhesive preferably has a rubber hardness of 70 or
smaller after adhered and hardened. In addition, it has such a
hardness contraction characteristic that the hardened contraction
after it is adhered and hardened does not substantially deform the
optical element 110. Preferably, degas from the adhesive does not
deteriorate the optical performance of the optical element 110.
[0044] Turning back to FIG. 1, the first spring part 150 is
provided among three support parts 130, and mounted on the
retaining member 120. The second spring part 160 is provided at a
side opposite to the first spring part 150 with respect to the
optical element 110.
[0045] The deteriorated optical performance of the optical element
110 caused by degas from the adhesive includes deteriorated
transmittance due to adhesions of degas onto the optical element
110 and a reaction product between the degas and exposure light
onto the optical element 110.
[0046] The deteriorated transmittance lowers, for example, the
throughput of the exposure apparatus, and the instant embodiment
uses Three Bond Co.'s TB1230 as the adhesive that does not cause
such influence.
[0047] The adjustment unit 130 controls a surface shape of the
optical element 110 placed on the retaining member 120. Three
adjustment units 130 are provided at three points at regular
intervals in the circumferential direction, and each point is a
midpoint between two adjacent support parts 122 in the retaining
member 120 for holding the optical element 110.
[0048] FIG. 3 illustrates a schematic sectional view of the
adjustment unit 130. The adjustment unit 130 includes, as shown in
FIG. 3, a hook 131, an elastic member 132, and a spacer 133, and a
strain gauge 134.
[0049] The hook 131 contacts bottom surfaces of the optical element
131 and the retaining member 120, and transmits a force to deform
the optical element 110 into a desired shape.
[0050] The elastic member 132 generates a force to deform the
optical element 110 into a desired shape. The instant embodiment
uses a tension spring for the elastic member 132, but can use a
flat spring and other spring element instead.
[0051] The spacer 133 is attached to the retaining member 120. One
of both ends of the spacer 133 is connected to the spacer 133 on
the retaining member 120, and the other is connected to the hook
131. An adjustment of a spacer 133's height would be able to create
a desired force in the elastic member 132. The generated force in
the elastic member 132 is transmitted to the optical element 110
through the hook 131, providing the optical element 110 with a
desired deformation amount.
[0052] The strain gauge 134 is adhered or fixed onto the top and
bottom surfaces of the hook 131 in FIG. 3. The instant embodiment
provides the strain gauge 134 to accurately know a force
transmitted from the hook 131 to the optical element 110. The
instant embodiment assumes that a deformation amount applied to the
optical element 110 is too minute to be measured, and replace the
deformation amount with a force that applies a desired
deformation.
[0053] The adhesive used to adhere and fix the strain gauge 132
preferably emits less degas to maintain the optical performance.
The less elastic adhesive is preferable so that a minute strain on
a measurement surface or a contact surface with the hook 131
transmits to the strain gauge 134.
[0054] The strain gauge 134 can measure a finer deformation amount,
as shown in FIG. 4, when plural strain gauges 134a and 134b are
adhered and fixed onto the top and bottom surfaces of the book 131.
Here, FIG. 4 is a schematic sectional view of another example of
the adjustment unit 130 shown in FIG. 1.
[0055] FIG. 5 is a circuitry of a bridge circuit that uses the
strain gauge 134. Referring to FIG. 5, the strain gauge 134
includes a bridge circuit with a resistor R of 120 .OMEGA., and
reads an output from a converter (not shown) that is connected to
the strain gauge 134. The load applied to the optical element 110
is detectable with precision by previously obtaining a relationship
between values from the strain gauge 134 and forces applied to the
retaining member 120.
[0056] A description will now be given of a mechanism of the bridge
circuit. R is fixed resistance, and Rm.sub.2 is the strain gauge
134 adhered and fixed onto the lower part of the hook 131. An
output voltage e.sub.O from the bridge circuit is expressed by
Equation 1 below, where E is a predetermined bridge voltage,
K.sub.S is a gauge ratio of the strain gauge, and .epsilon..sub.O
is a strain amount: 1 e 0 = E 2 K s 0 ( 1 )
[0057] For example, the output voltage e.sub.O becomes 1 mV when
strain amount .epsilon..sub.O=100.times.10.sup.-6, the bridge
voltage E is 10 V, the gauge ratio K.sub.S=2. Thus, the output
voltage can be obtained according to strain amounts of the strain
gauge. Equation 1 indicates that there is a linear relationship
between the strain amount and the output voltage. In order to more
accurately measure the bridge circuit shown in FIG. 5, a bridge
circuit that includes four strain gauges can be used by replacing
two fixed resistors R with the strain gauge 134 at the side of the
retaining member 120 in FIG. 120. Thereby, the output-voltage
sensitivity becomes doubled to the strain amount for more precise
measurements.
[0058] A description will now be given of a procedure of adhering
the optical element 110 onto the retaining member 120. First, the
optical element 110 is placed on three support parts 122 in the
retaining member 120. Center positions of the optical element and
the retaining member 120 have certain circularity so that the joint
parts 124 have a uniform thickness between the optical element 110
and the retaining member 120.
[0059] After the optical element 110 is placed on the support parts
122 in the retaining member 120, centers of the optical element 110
and the retaining member 120 are aligned with each other so that a
clearance amount between the optical element 110 and the retaining
member 120 is uniform and constant over the entire circumference.
When the clearance amount between the optical element 110 and the
retaining member 120 becomes uniform and constant, the adhesive is
injected into the joint part 124 through a dispenser or the like
that can control an injection amount. An optimization of an amount
of the adhesive to be injected into the joint part 124 would be
able to control the mechanical rigidity of the joint part 124 as an
elastic member that connects the optical element 110 and the
retaining member 120 to each other.
[0060] The instant embodiment needs to deform the joint part 124
when the retaining member 120 transmits a force to the optical
element 110, in providing the optical element 110 with a desired
deformation amount through the adjustment unit 130. Control over
deformations of the joint part 124 needs to control an elastic
constant of the joint part 124 as an elastic member to a
predetermined value. In using a flat spring to fix the optical
element 110 onto the retaining member 120, the elastic constant of
the flat spring can be controlled.
[0061] After the optical element 110 is adhered and fixed onto the
retaining member 120, the adjustment units 130 arranged at three
points in the retaining member 120 are used to apply forces to the
optical element 110 by changing a thickness of the spacer 133 to a
desired thickness. The force applied to the optical element 110 can
be accurately read by monitoring the output of the strain gauge
134. Thus, a desired displacement can be provided each of the three
points on the optical element 110. The provisions of displacements
to the optical element 110, which cancel out 30 self-weight
deformations caused when the optical element 110 is held by the
retaining member 120, can reduce a deformation of the optical
element 110 which negatively affects its optical performance.
[0062] Use of the retainer 100, for example, to hold an optical
element in a projection optical system in an exposure apparatus
would be able to reduce-influence of the self-weight deformation of
the optical element that has been supported at three points. This
structure can retain the optical element while reducing the
self-weight deformation of the large aperture optical element for a
high NA exposure apparatus, and exposure performance significantly
increases without being negatively affected by the deformed optical
element.
[0063] Referring now to FIGS. 6 to 8, a description will be given
of an adjustment unit 130A as a variation of the adjustment unit
130. FIG. 6 is a schematic sectional view showing one example of
the adjustment unit 130A as a variation of the adjustment unit 130.
The adjustment unit 130 adjusts the force in the elastic member 132
by adjusting the height of the spacer 133, whereas the adjustment
unit 130A adjusts the force in the elastic member 132 outside the
retaining member 120. The adjustment unit 130A includes, as shown
in FIG. 6, the hook 131, the elastic member 132, the strain gauge
134, and an adjustment screw 135.
[0064] The adjustment screw 135 has a cone, spiral part 135a at its
top, and one end of the elastic member 132 is attached to the
spiral part 135a. As the adjustment screw 135 rotates, the
adjustment screw 135 proceeds and the end of the elastic member 132
attached to the spiral part 135a can change its position in a
longitudinal direction in FIG. 6. Therefore, as the adjustment
screw 135 is rotated from the outside of the retaining member 120,
the elastic member 132 can be adjusted its length to apply a
desired force to the optical element 110. Since the adjustment
screw 135 uniformly adjust the length of the elastic member 132
around a screw rotating angle, an axis 135b of the adjustment screw
135 connected to the retaining member 120 and an axis 135a of the
spiral part 135a must be co-axial. The spiral part 135a even when
having a cone shape in the adjustment screw 135 can obtain similar
functions by variably sliding the elastic element 132 in the
longitudinal direction.
[0065] When the hook 131 that contacts the optical element 110
offsets in the horizontal direction, the load to the optical
element 110 can vary, throw off balance of the optical element 110,
deteriorate the surface shape or cause inconsistency with a design
value. Accordingly, as shown in FIG. 7, a fixing member 136 is
attached to a contact area 128 between the retaining member 120 and
the hook 131 to fix the hook 131 and prevent a horizontal offset.
Here, FIG. 7 is a schematic sectional view showing one example of
the adjustment unit 130A that has the improved contact area 128
between the retaining member 120 and the hook 131.
[0066] The fixing member 136 is fixed onto the hook 131 in the
horizontal direction and made, for example, of a thin plate. A
thickness of the fixing member 136 is designed such that the
bending stress of the fixing member 136 is much lower than the
force generated in the elastic member 132 so as not to negatively
affect control over a shape of the optical element 100.
[0067] As shown in FIG. 8, the hook 131 can have a fixed position
with a reduced or eliminated horizontal offset by forming an acute
shape at the end 131a of the hook 131 which contacts the retaining
member 120, and providing the area 128 in the retaining member 120
for receiving the end 131a of the hook 131 (i.e., the contact area
between the retaining member 120 and the hook 131), with a cone or
V-shaped groove 128a. In this case, the end 131a of the hook 131
and the groove 128a in the retaining member 120 are preferably
sharper within a permissible rigidity range so that the hook 131
does not shift in the horizontal direction even when receiving a
large lateral force. Here, FIG. 8 is a sectional view showing
another example of the adjustment unit 130A that has the improved
contact area 128 between the retaining member 120 and the hook
131.
[0068] The adjustment unit 130A adjusts the force to be generated
by the elastic member 132 from the outside of the retaining member
120 or a length of the elastic member 132, and controls 30
deformations of the optical element 110. Therefore, the adjustment
unit 130A can control a shape of the optical element 110 from the
outside, for example, of a lens barrel for the optical element
110.
[0069] The lens barrel as a whole includes the wave front
aberration occur due to the 3.theta. deformations of the plural
optical elements 110, assembly errors, homogeneity, etc. Control
over the force generated in the elastic member 132 in the
adjustment unit 130A can minimize the wave front aberration. It is
conceivable to compulsorily increase the 3.theta. deformations of
the plural optical elements 110 so as to cancel out the wave front
aberration. A combination of independent adjustments of 3.theta.
deformations of plural optical elements 110 would vary the wave
front aberration can change in a wider range. The optical element
110 that is highly sensitive to the wave front aberration thus can
adjust the wave front aberration in a wide range. The optical
element 110 that is not so sensitive to the wave front aberration
thus can adjust the wave front aberration with high resolution.
[0070] An installment of electromotive equipment, such as an
actuator, for rotating the adjustment screw 135 would rotate the
adjustment screw 135 without direct access to inside of the lens
barrel. Thereby, the lens barrel does not have to include a hole
for access to the inside of the lens barrel, and the adjustment
screw 135 can be rotated with high precision and resolution. Thus,
the optical element 110 can be adjusted to a desired shape by
monitoring an output of the strain gauge 134 adhered and fixed onto
the hook 131, and by controlling a rotation of the adjustment screw
135 based on this value.
[0071] A description will now be given of a method for actively
controlling a shape of the optical element 110 by feedback control
using the retainer 100. FIG. 9 is a flowchart of an adjustment
method 1000 for adjusting a shape of the optical element 110 held
by the retainer 100.
[0072] When the optical element 110 deforms its shape, a change of
an output of the strain gauge 134 attached to the retaining member
120 provides a deformed shape of the optical element 110 (step
1002). Then, the force that is necessary to change the deformed
optical element 110 into a desired shape and to be applied to the
optical element 110 from the elastic member 132 is calculated (step
1004). A length of the elastic member 132 is adjusted so that the
elastic member 132 creates the calculated force (step 1006). The
calculated force is applied to the optical element 110 (step 1008).
After the optical element 110 is forced, the strain gauge 134
measures a shape of the optical element 110, and it is determined
whether the optical element 110 has a desired shape (step 1010).
When the optical element 110 has a desired shape, the control over
the shape of the optical element 110 ends (step 1012). When the
optical element 110 does not have a desired shape, the procedure
subsequent to the step 1004 is repeated. A relationship between a
shape of the optical element 110 and an output of the strain gauge
134 has been previously obtained. The feedback control over the
shape of the optical element 110 is thus available by using the
strain gauge 134 as a detector of a shape of the optical element
110.
[0073] The optical element 110 generates wave front aberration due
to errors, such as a self-weight deformation, a surface shape
error, and homogeneity, and this wave front aberration should fall
within a desired range. In this case, a wave front aberration
measuring apparatus that directly measures the wave front
aberration of the optical element 110 detects the wave front
aberration in the optical element 110 (step 1014), and adjusts a
length of the elastic member 132 so that the detected wave front
aberration falls within a permissible range for feedback control
(step 1006).
[0074] The exposure apparatus mounted with the retainer 100 that
holds the optical element 110 uses means for detecting a wafer
pattern to always detect a wafer (step 1016), and adjust a length
of the elastic member 132 based on the detected wafer information
for feedback control (step 1006). This feedback control can realize
an active lens or mirror for always controlling a shape of the
optical element 110. As a result, a wafer pattern is always
detected so as to obtain a desired wafer pattern, and maintained
within a desired standard.
[0075] Alternatively, there is a processor that measures a strain
amount of the hook 131 and calculates a relationship between the
measured amount and a force to be applied to the optical element
when the strain amount occurs, or a memory that stores the
relationship. In this case, it is determined whether or not the
strain amount of the hook 131 is within a permissible range, and if
it is determined within the permissible range, the force to be
applied to the optical element is maintained.
[0076] While FIGS. 1 and 2 arrange three adjustment units 130 and
strain gauges (as a detector) 134 at intervals of three support
parts 122, but the present invention is not limited to this
configuration. Presumably, the optical element 110 similarly
deforms in three spaces among the three support parts. That is, if
a deformation amount of the optical element in one space or a
measured strain amount by the strain gauge in one space is known, a
deformation amount of the optical element or a measured strain
amount by the strain gauge in the other remaining spaces can be
inferred easily. Therefore, the strain gauge is arranged in at
least one space among three spaces among the three support parts,
and the adjustment units in other spaces may control, based on the
strain measured by the strain gauge, forces applied to the optical
element.
[0077] Referring now to FIG. 10, a description will be given of a
retainer 101A as a variation of the retainer 100. FIG. 10 is a
schematic perspective view of the retainer 100A as a variation of
the retainer 100.
[0078] The retainer 100A measures, as shown in FIG. 10, a
deformation of the optical element 110 by adhering a strain gauge
134 directly to the optical element 110. An adhesion of the strain
gauge 134 to a top or bottom surface of the optical element 110,
which is held by the retaining member 120, outside the effective
radius, enables the strain gauge 134 to detect a deformation of the
optical element 110. The optical element 110 is retained by
three-point supporting, an adhesion of the entire peripheral, a
mechanical retaining method, or the like.
[0079] A description will be given of use of the strain gauge 134
adhered onto the optical element 110. First, the strain gauge 134
is adhered onto the optical element 110 before the optical element
110, which has a processed surface shape within an optical design
value standard, is adhered and fixed onto the retaining member 120.
Then, an output value is read out from the strain gauge 134.
[0080] Next follows a mount of the optical element 110 onto the
retaining member 120, and an arrangement of the adjustment unit
130. An output value is read out from the strain gauge before or
after the final stage where the retainer 100 that retains, the
optical element 100 is installed into the apparatus. A deformation
(or the optical element 110's deformation by its own weight) as a
result of holding the optical element 110 can be measured by
reading a difference between the output value at that time and the
initial value (i.e., an output value before the optical element 110
is not adhered or fixed onto the retaining member 120).
[0081] A deformation as a result of holding the optical element 110
can be cancelled out when the adjustment unit 130 adjusts the
output value of the strain gauge 134 close to the initial
value.
[0082] A description will be given of another use of the strain
gauge 134 adhered and fixed onto the optical element 110. First,
the strain gauge 134 is adhered and fixed onto the optical element
110 held by the retaining member 120. The adjustment unit 130
varies stepwise a shape of the optical element 110, and measures a
deformation amount of the optical element 110 for each stage from
output values from the strain gauge 134. This previously obtained
correlation between a deformation amount of the optical element 110
and an output value of the strain gauge 134 would be able to deform
the optical element 110 into a desired shape only through
adjustments by the adjustment unit 130 from the outside of the lens
barrel into which the retainer 100 holding the optical element 110
is incorporated. This method can be realized without considering
strain gauge 134's attachment errors, and adjustment unit 130's
assembly and processing errors.
[0083] The strain gauge 134 can similarly be adhered directly onto
the optical element 110 even as a mirror. The strain gauge 134
adhered onto a rear surface 110b opposite to a reflective surface
110a of the optical element 110 as a mirror, as shown in FIG. 11,
would be able to detect large strains for precise measurements.
FIG. 11 is a schematic sectional view of the retainer 100A when the
optical element 110 is a mirror.
[0084] Referring now to FIG. 12, a description will be given of the
projection optical system 230 to which the inventive retainer 100
is applied and the exposure apparatus 200 having the same. Here,
FIG. 12 is a schematic block diagram of the illustrative exposure
apparatus 200 of the instant embodiment. The exposure apparatus 200
includes, as shown in FIG. 12, an illumination apparatus 210 for
illuminating a mask 220 which forms a circuit pattern, a projection
optical system 230 that projects diffracted light created from the
illuminated mask pattern onto a plate 240, and a stage 245 for
supporting the plate 240.
[0085] The exposure apparatus 200 is a projection exposure
apparatus that exposes onto the plate 240 a circuit pattern created
on the mask 220, e.g., in a step-and-repeat or a step-and-scan
manner. Such an exposure apparatus is suitable for a sub-micron or
quarter-micron lithography process, and this embodiment exemplarily
describes a step-and-scan exposure apparatus (which is also called
"a scanner"). "The step-and-scan manner", as used herein, is an
exposure method that exposes a mask pattern onto a wafer by
continuously scanning the wafer relative to the mask, and by
moving, after a shot of exposure, the wafer stepwise to the next
exposure area to be shot. "The step-and-repeat manner" is another
mode of exposure method that moves a wafer stepwise to an exposure
area for the next shot every shot of cell projection onto the
wafer.
[0086] The illumination apparatus 210 illuminates the mask 220
which forms a circuit pattern to be transferred, and includes a
light source unit 212 and an illumination optical system 214.
[0087] The light source unit 212 uses as a light source, for
example, as ArF excimer laser with a wavelength of approximately
193 nm, a KrF excimer laser with a wavelength of approximately 248
nm, and F.sub.2 excimer laser with a wavelength of approximately
153 nm, but the a type of laser is not limited to excimer laser and
a YAG laser may be, for example. Similarly, the number of laser
units is not limited. F.sub.2 laser with a wavelength of about 157
nm and an extreme ultraviolet ("EUV") light source with a
wavelength between about 10 nm and about 20 nm are also applicable.
For example, two independently acting solid lasers would cause no
coherence between these solid lasers and significantly reduces
speckles resulting from the coherence. An optical system for
reducing speckles may swing linearly or rotationally. When the
light source unit 212 uses laser, it is desirable to employ a beam
shaping optical system that shapes a parallel beam from a laser
source to a desired beam shape, and an incoherently turning optical
system that turns a coherent laser beam into an incoherent one. A
light source applicable to the light source unit 212 is not limited
to a laser., and may use one or more lamps such as a mercury lamp
and a xenon lamp.
[0088] The illumination optical system 214 is an optical system
that illuminates the mask 220, and includes a lens, a mirror, a
light integrator, a stop, and the like, for example, a condenser
lens, a fly-eye lens, an aperture stop, a condenser lens, a slit,
and an image-forming optical system in this order. The illumination
optical system 214 can use any light whether it is axial or
non-axial light. The light integrator may include a fly-eye lens or
an integrator formed by stacking two sets of cylindrical lens array
plates (or lenticular lenses), and be replaced with an optical rod
or a diffractive element. The aperture stop can include an annular
illumination stop and a quadrupole illumination stop for modified
illumination that improves resolution. The inventive retainer 100
may be used to hold the optical element, such as a lens in the
illumination optical system 214.
[0089] The mask 220 is made, for example, of quartz, forms a
circuit pattern (or an image) to be transferred, and is supported
and driven by a mask stage (not shown). Diffracted light emitted
from the mask 220 passes the projection optical system 230, thus
and then is projected onto the plate 240. The mask 220 and the
plate 240 are located in an optically conjugate relationship. Since
the exposure apparatus 200 of this embodiment is a scanner, the
mask 220 and the plate 240 are scanned at the speed ratio of the
reduction ratio of the projection optical system 230,
thus-transferring the pattern on the mask 220 to the plate 240. If
it is a step-and-repeat exposure apparatus (referred to as a
"stepper"), the mask 220 and the plate 240 stand still in exposing
the mask pattern.
[0090] The projection optical system 230 may use an optical system
solely including a plurality of lens elements, an optical system
including a plurality of lens elements and at least one concave
mirror (a catadioptric optical system), an optical system including
a plurality of lens elements and at least one diffractive optical
element such as a kinoform, and a full mirror type optical system,
and so on. Any necessary correction of the chromatic aberration may
use a plurality of lens units made from glass materials having
different dispersion values (Abbe values), or arrange a diffractive
optical element such that it disperses in a direction opposite to
that of the lens unit.
[0091] The inventive retainer 100 may be used to hold the optical
element, such as a lens in the projection optical system 230. The
retainer 100 is connected to the lens barrel 232 in the projection
optical system 230 through the absorptive member 122 that is
provided on the retaining member 120, as shown in FIG. 13, and may
absorb radial deformations. This structure may prevent the
retaining member 120 from decentering due to a relative
displacement between the lens barrel 232 and the retaining member
120, which relative displacement results from different
coefficients of linear expansion between them, when the temperature
environment changes, for example, in carrying the apparatus. Here,
FIG. 13 is a schematic, partial sectional, perspective view in the
lens barrel 232 in the exposure apparatus 200.
[0092] Due to the above structured retainer 100, the projection
optical system 230 may achieve desired optical performance by
reducing the aberration that results from the deformation and
positional offset of the optical element 110 which otherwise
deteriorates imaging performance.
[0093] The plate 240 is an object to be exposed such as a wafer and
a liquid crystal plate, and photoresist is applied onto it. A
photoresist application step includes a pretreatment, an adhesion
accelerator application treatment, a photoresist application
treatment, and a pre-bake treatment. The pretreatment includes
cleaning, drying, etc. The adhesion accelerator application
treatment is a surface reforming process so as to enhance the
adhesion between the photo-resist and a base (i.e., a process to
increase the hydrophobicity by applying a surface active agent),
through a coat or vaporous process using an organic film such as
HMDS (Hexamethyl-disilazane). The pre-bake treatment is a baking
(or burning) step, softer than that after development, which
removes the solvent.
[0094] The stage 245 supports the plate 240. The stage 240 may use
any structure known in the art, and a detailed description of its
structure and operation is omitted. The stage 245 may use, for
example, a linear motor to move the plate 240 in XY directions. The
mask 220 and plate 240 are, for example, scanned synchronously, and
the positions of the stage 245 and a mask stage (not shown) are
monitored, for example, by a laser interferometer and the like, so
that both are driven at a constant speed ratio. The stage 245 is
installed on a stage stool supported on the floor and the like, for
example, via a damper, and the mask stage and the projection
optical system 230 are installed on a lens barrel stool (not shown)
supported, for example, via a damper to the base frame placed on
the floor.
[0095] In exposure, light emitted from the light source 212, e.g.,
Koehler-illuminates the mask 220 via the illumination optical
system 214. Light that passes through the mask 220 and reflects the
mask pattern is imaged onto the plate 240 by a magnification of the
projection optical system 230, such as 1/4 and 1/5. The projection
optical system 230 and/or the illumination optical system 214 used
for the exposure apparatus 200 include an optical element held by
the inventive retainer 100, and reduce the deformation and the
aberration resulting from the positional offset of the optical
element, thus being able to provide high-quality devices (such as
semiconductor devices, LCD devices, photographing devices (such as
CCDs, etc.), thin film magnetic heads, and the like).
[0096] Referring now to FIGS. 14 and 15, a description will be
given of an embodiment of a device fabrication method using the
above mentioned exposure apparatus 200. FIG. 14 is a flowchart for
explaining how to fabricate devices (i.e., semiconductor chips such
as IC and LSI, LCDs, CCDs, and the like). Here, a description will
be given of the fabrication of a semiconductor chip as an example.
Step 1 (circuit design) designs a semiconductor device circuit.
Step 2 (mask fabrication) forms a mask having a designed circuit
pattern. Step 3 (wafer making) manufactures a wafer using materials
such as silicon. Step 4 (wafer process), which is also referred to
as a pretreatment, forms actual circuitry on the wafer through
lithography using the mask and wafer. Step 5 (assembly), which is
also referred to as a post-treatment, forms into a semiconductor
chip the wafer formed in Step 4 and includes an assembly step
(e.g., dicing, bonding), a packaging step (chip sealing), and the
like. Step 6 (inspection) performs various tests for the
semiconductor device made in Step 5, such as a validity test and a
durability test. Through these steps, a semiconductor device is
finished and shipped (Step 7). FIG. 15 is a detailed flowchart of
the wafer process in Step 4. Step 11 (oxidation) oxidizes the
wafer's surface. Step 12 (CVD) forms an insulating film on the
wafer's surface. Step 13 (electrode formation) forms electrodes on
the wafer by vapor disposition and the like. Step 14 (ion
implantation) implants ion into the wafer. Step 15 (resist process)
applies a photosensitive material onto the wafer. Step 16
(exposure) uses the exposure apparatus 200 to expose a circuit
pattern on the mask onto the wafer. Step 17 (development) develops
the exposed wafer. Step 18 (etching) etches parts other than a
developed resist image. Step 19 (resist stripping) removes disused
resist after etching. These steps are repeated, and multi-layer
circuit patterns are formed on the wafer. Use of the fabrication
method in this embodiment helps fabricate higher-quality devices
than conventional. Thus, the device fabrication method using the
exposure apparatus 200, and resultant devices constitute one aspect
of the present invention.
[0097] Further, the present invention is not limited to these
preferred embodiments and various variations and modifications may
be made without departing from the scope of the present invention.
For example, the inventive retainer may be used to hold various
optical elements, such as a lens, mirror, and filter. The inventive
retainer may be used to hold a mask and a wafer.
[0098] The inventive retainer provides a strain gauge for detecting
a shape of an optical element on the optical element or a retaining
member that directly holds the optical element, and enables the
shape of the optical element to be easily recognized and adjusted
based on an output of the strain gauge. Therefore, the inventive
retainer can realize a high-resolution projection optical system
with desired optical performance by reducing aberration due to a
deformation of an optical element which otherwise deteriorates the
imaging performance.
* * * * *